Logarithmic discriminator



Aug. 23, 1955 R". u. SIMS 2,716,212

LOGARITHMIC DISCRIMINATOR Filed Aug. 12, 1952 VOLTS OUTPUT Illillllllillllll! PHIHII! liilllll FREQUENCY, CPS- INVENTOR J, I E, (RAYMOND U.SIMS f 1/ Z. CLQM/PZ ATTORNEYS United States Patent LOGARITHMICDISCRIMINATOR Raymond U. Sims, Pittsburgh, Pa., assiguor, by mesneassignments, to the United States of America as represented by theSecretary of the Navy Application August 12, 1952, Serial No. 304,036

2 Claims. (Cl. 321-27) The present invention relates to measuringinstruments, more particularly to instruments in which the indicationgiven is substantially a linear function of the logarithm of thequantity measured.

Previous logarithmic measuring devices have been objectionable becausethey are comparatively more complicated thereby requiring morecomponents and because they have a relatively low conversion efficiency.An illustration of previous type circuits is shown in the publicationElectronics June 1949, pages 96 and 97, in an article by Harry M. Crain,entitled, Low frequency discriminator. Many others of the circuitsheretofore proposed have been relatively unstable or difiicult tomanufacture with the desired overall characteristics.

An object, therefore, of the present invention is to provide alogarithmic discriminator capable of producing a typical discriminator 8curve on a logarithmic frequency axis for greater range in frequency.

Another object is to provide a logarithmic discriminator the output ofwhich is a substantially linear function of the logarithm of thefrequency input.

A further object is to provide a logarithmic discriminator that utilizesrelatively few components and which is convenient to manufacture.

Still another object of the present inventionis to provide a logarithmicdiscriminator that has a comparatively high conversion efliciency.

A final object of the present invention is to provide a logarithmicdiscriminator that is stable in its characteristics.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. l is a circuit diagram of one embodiment of the invention; and

Fig. 2 presents curves illustrating the basic principle of theinvention.

Referring now to the drawing wherein like reference characters designatelike or corresponding parts throughout the several views, there is shownin Fig. l, the circuit diagram of the logarithmic discriminatorincluding the variable frequency source 11 which energizes condenser C12and resistor R13 in series, which form a branch; The voltage developedacross resistor R as applied to plate 14 of diode rectifier 15, whichhas its cathode 16 feeding into the cathode load circuit 17, comprisingresistor R13 and condenser C19 in parallel. Resistor R20 and condenserC21, which form a second branch, are energized in series by source 11.The voltage developed across condenser C2 is applied to plate 22 ofdiode rectifier 23. Cathode 24 of diode 23 feeds into the common cathodeload circuit 17. The voltage developed across the cathode load circuit17 is represented by (217. Condenser C2 and resistor R26, in series,form a third branch, and are similarly energized by variable frequencysource 11. The voltage developed across resistor R26 is applied to plate27 of diode rectifier 28. Cathode 29 of the diode 28 is connected tocathode load circuit 30, comprising condenser C31 and resistor R32 inparallel. In like manner resistor R33 and condenser C34, in series, forma fourth branch, and are energized by the variable frequency source 11.The voltage developed across condenser C34 is applied to the plate 35 ofdiode rectifier 36. Cathode 37 of diode 36 feeds into common cathodeload circuit 30. The voltage developed across the cathode load circuit30 is represented by e30. The output voltage measured at outputterminals 38 will be equal to em minus e17.

In deriving the relations that exist in the circuit of the presentinvention the following symbols will be used:

E=voltage applied to the circuit e=resultant voltages R=resistanceC=capacitance t=time f=frequency w= angular frequency (21rf) =phaseangle of impedance With the foregoing in mind and referring to theaccompanying figures the theory of operation of the device is asfollows:

When a sinusoidal voltage of constant amplitude and variable frequencyis applied to a series circuit, the voltage across the resistive elementis ERwC sin (wt +2) WW while the voltage across the condenser is E sin(wrap-5) l Let us consider two such series circuits, as condenser C12and resistor R13 and resistor R20 and condenser C21, in Fig. 1, both ofwhich are energized by voltage source 11. Let diodes 15 and 23 beconnected to rectify the voltage developed across resistor R13 andcondenser C21, respectively, and thereafter feed rectified current intothe parallel circuit 17, comprising resistor R12 and condenser C19. Letus assume that the angular frequency w of the voltage or signal, Eapplied at input terminals 11 varies according to the followingconditions:

If w is initially less than 1 R2002; (3) and R20C21 is greater thanR3C12 (4) then as w increases, the reactance of C21 decreases, there bydecreasing the voltage applied to plate 22 of diode 23. This decreasingvoltage causes the conduction of diode tube 23 to diminish, therebylowering the voltage 21? developed across the parallel circuit 17. As wincreases e17 decreases until under which conditions the voltagesdeveloped across R13 and C21 will be equal thereby causing tubes 15 and23 to conduct current equally. When w increases still fur ther thevoltage developed across R13 will increase and become greater than thevoltage across C21, thereby causing an increase in conduction of tube 15and resulting in an increasing voltage 617 at the parallel circuit 17.The relations described above can be verified by examining Equations 1and 2. Fig. 2 graphically illustrates this action wherein the voltagesacross capacitor C31 and resistor R13 are represented on the graph as221 and en both of which combine to produce the voltage cm across theparallel circuit 17.

The circuit constants of R2sC25 and R33Cs4 are similarly chosen so thatthe voltage across the parallel circuit 30 as represented by the curvecan plotted against the logarithm of frequency, is identical to 617,except that it is displaced along the abscissa scale. It can be seenfrom Fig. 2, that voltage curve ear; is composed of the voltage curvese34, representing the voltage across condenser C34, and 22s representingthe voltage across resistor R2s- The output voltage represented by curvee38 is obtained by taking the difference between eat) and en, at outputterminals 38. The separation of angles A and C, of Fig. 2, should besuch that angle B, the crossover of 617 and em, is about the same asangles A and C. Satisfying these conditions assures that the outputvoltage eat; is zero at the point of inflection of the resulting 8curve.

It can be seen from Fig. 2 that the logarithmic variation of frequency wproduces a linear variation of voltage on curve em, from approximately2000 C. P. S. to 50,000 C. P. S. By selecting circuit constants in themanner described above the straight line portion of the S curve deviatesvery little from the ideal over a frequency range of 25 to 1.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

I claim:

1. In an electronic discriminator a pair of input terminals adapted tobe supplied with electrical energy of variable frequency; a firstcondenser and resistor series circuit energized by said electricalenergy; a first diode rectifier adapted to rectify the voltage acrossthe first resistor; a second resistor and condenser series networkenergized by said electrical energy; a second diode rectifier adapted torectify the voltage across the second condenser; said first and seconddiode rectifier having their cathodes connected together; a firstresistor-condenser parallel circuit to store the rectifier output signaland having an output terminal; a third condenser and resistor seriescircuit energized by said electrical energy; a third diode rectifieradapted to rectify the voltage across the third resistor; a fourthresistor and condenser series network energized by said electricalenergy; a fourth rectifier adapted to rectify the voltage across thefourth condenser; said third and fourth diode rectifiers having theircathodes connected together; a second resistor-condenser parallelcircuit to store the rectifier output signal and having an outputterminal; the difierence in potential between the output terminals beingsubstantially a linear function of the logarithm of the frequency of theelectrical energy.

2. A logarithmic discriminator comprising a first branch having acondenser and resistor connected in series, a second branch having aresistor and condenser connected in series, a third branch having acondenser and resistor connected in series, and a fourth branch having aresistor and a condenser connected in series, the resistors andcondensers of said branches being connected in the order named, and thebranches being connected in parallel and adapted to have appliedthereacross an A. C. signal of varying frequency, means for producing afirst unidirectional voltage which is substantially equal to theamplitude of the larger A. C. voltage developed across the resistor ofsaid first branch or the condenser of the second branch, and means forproducing a second unidirectional voltage which is substantially equalto the amplitude of the larger A. C. voltage developed across theresistor of said third branch or the condenser of the fourth branch, thevalues of the resistors and condensers in said branches being such thatthe difference between said first and second unidirectional voltages issubstantially a linear function of the logarithm of the frequency of theA. C. signal over a substantially wide range.

References Cited in the file of this patent UNITED STATES PATENTS2,491,921 Hepp Dec. 20, 1949 2,522,893 Purington Sept. 19, 19502,567,194 Earp Sept. 11, 1951

